Process for measuring the concentration of gases

ABSTRACT

A process is provided for measuring a gas concentration with at least one thermal measuring element. The thermal measuring element is operated by pulses. A measured value of the gas concentration in the environment of the measuring element is obtained from the evaluation of the response of the measuring element to at least one single pulse by determining transient states of the thermal measuring element. From this the measured value of the gas concentration in the environment of the measuring element can be derived, during the imposed pulse based on electric measured variables.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 ofGerman Patent Application DE 10 2005 024 394.0 filed May 27, 2005, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention pertains to a process for measuring theconcentration of gases by means of thermal measuring elements.

BACKGROUND OF THE INVENTION

Processes for measuring the concentration of gases with thermalmeasuring elements are known per se. As a rule, the thermal behavior ofthe measuring element is observed by evaluating electric measuredvariables. The thermal behavior of the measuring element depends indifferent ways on the concentrations of different gases in theenvironment of the measuring element.

It is known that such processes can be carried out with so-calledpellistors, which are partially catalytically active (U.S. Pat. No.4,457,954, GB 2083630, U.S. Pat. No. 4,583,070). Two pellistors areusually used in case of the catalytic principle of measurement, onepellistor being prepared catalytically, while the second pellistor doesnot have this catalytic preparation.

The behavior of the catalytically prepared pellistor to change itsresistance, which is characteristic of the gas to be detected, comparedto the second, unprepared pellistor can be evaluated by means of aprior-art resistance bridge.

Various operating processes, such as constant-current, constant-voltageor constant-resistance processes, are known per se. The drawback ofthese processes in continuous operation is the high power consumption,which may be between 250 mW and 700 mW.

The following patent specifications U.S. Pat. No. 4,861,557, DE 4330603and DE 3131710 describe processes with a pellistor bridge. The drawbacksof such processes are the complicated apparatus required for twomeasuring elements, the actuation thereof in a continuous mode ofoperation, and the high power requirement associated therewith.

A process with low power consumption is known, in which combustiblegases are detected with only one measuring element (EP 0234251 A1). Thegas concentration is determined in two stationary measurement phasesaccording to this process.

The drawbacks of this process are that interfering environmentaleffects, e.g., temperature, pressure or moisture, are not compensated,and the power consumption is still very high for the operation in twomeasurement phases.

It is known that thermal measuring elements can be operated cyclically,in which case three different phases of operation alternate regularly(DE 69020343 T2). A heating phase is first carried out, during which themeasuring element is heated to a preset resistance. This is followed bythe measurement phase, during which the measuring element is maintainedat a constant resistance value. This is followed by a rest phase, duringwhich the measuring element is adjusted to a static resistance. A Ptair-core coil with a wire diameter of 80 μm is mentioned as themeasuring element. The working temperature of the resistor element istherefore selected to be such that the temperature set point of the Ptcoil is in the range of 570° C. to 1,100° C. By contrast, the use ofpellistors was rejected, mainly because of the thermal inertia of thepellistor beads and the limited long-term stability. The drawback ofthis process is that the Pt air-core coils also have an excessively lowlong-term stability at such high operating temperatures and a relativelyhigh power consumption is also associated with the high operatingtemperature.

Furthermore, it is known that the impression of an excitation functionand evaluation of the response function can be used to determine thepercentage of combustible gases in a gas mixture as well as to classifythem according to gas classes (DE 4311605 C1). Depending on the selectedfunction, the effort needed for control and evaluation may be high.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a process that makesit possible with little effort to carry out gas concentrationmeasurements with the smallest possible amount of energy.

According to the invention, a process is provided for measuring the gasconcentration with at least one thermal measuring element, which isoperated by pulses. A measured value of the gas concentration in theenvironment of the measuring element is obtained from the evaluation ofthe response of the measuring element to at least one single pulse bydetermining transient states of the thermal measuring element during theimposed pulse. From the transient states the measured value of the gasconcentration in the environment of the measuring element can bederived, by means of accessible electric measured variables.

The present invention is based on the fact that it is possible to supplythermal measuring elements with energy by pulses and to obtain ameasured value of the gas concentration in the environment of themeasuring element from the evaluation of the response of the measuringelement to the individual feed pulse. Transient states of the thermalmeasuring element are determined for this by means of accessibleelectric measured variables during an imposed pulse before a stable endvalue of the particular measured variable is reached. The measurement ispreferably performed periodically.

Transient states of thermal measuring elements are defined in the senseof the present invention primarily as physical or chemical properties ofthe thermal measuring elements that may appear briefly when thermalmeasuring elements are not in a thermal equilibrium. This happens, forexample, at times that are close to changes in an imposed power, whenthese changes take place so rapidly that quasi-stationary changes instate in the volume of the thermal measuring elements are practicallyruled out. This happens especially in case of pulsed energy supply.

Due to the technical improvement of the catalytic pellistor beads, thestability of their sensitivity has improved markedly compared to theearlier state of the art. In addition, pellistors are now known whosethermal mass is so small that they are suitable for transientmeasurements with short time constants. The process according to thepresent invention can, moreover, also be carried out with othergas-sensitive thermal measuring elements.

The thermal measuring element may advantageously be a catalyticallyactive pellistor. This has a catalytic layer that contains catalysts,for example, Rh, Pt, Pd or other catalytically active elements orcompounds of elements or combinations thereof. Advantageous carriermaterials, which are used to support the catalysts, are aluminum oxide,zirconium oxide, magnesium oxide or other carrier substances or mixturesthereof. It was found that catalytic reaction of gases to be detectedcan already take place at a temperature between 100° C. and 570° C. withpellistors that contain the above-mentioned materials. This is adecisive advantage concerning long-term stability and power consumption.

To carry out the process according to the present invention, acatalytically active pellistor is preferably to be used whenever achemical reaction of the gases being measured shall lead to a change inthe resistance of the thermal element.

However, the thermal measuring element may also be a catalyticallyinactive pellistor. This inactive pellistor is preferably likewiseheated for the measurement to a temperature between 100° C. and 570° C.

A catalytically inactive pellistor is preferably to be used wheneverthermal properties of the gas being measured shall lead to a change inthe resistance of the thermal element without a catalytic reaction beingnecessary.

The thermal element may advantageously be a combination of acatalytically active pellistor with an inactive pellistor, which aretemporarily heated to a temperature between 100° C. and 570° C.

The process according to the present invention can be carried out, inprinciple, with most thermal measuring elements, i.e., for example,microstructured elements, Pt 100 or air-core coils, whose measurableeffect is that a change in temperature leads to a change in the electricresistance.

The present invention comprises a process for measuring a gasconcentration with at least one thermal measuring element, which isoperated by pulses, and in which a measured value of the gasconcentration in the environment of the measuring element is obtainedfrom the evaluation of the response of the measuring element to at leastone single pulse, characterized in that transient states of the thermalmeasuring element, from which the measured value of the gasconcentration in the environment of the measuring element can bederived, are determined during the imposed pulse on the basis ofaccessible electric measured variables. The process according to thepresent invention comprises only two phases, a measuring phase and arest phase. A variable electric power P_(measurement) that is necessaryfor the measuring operation is fed in during the measurement phase(duration T_(measurement)). A variable or constant electric powerP_(rest) necessary for the rest operation of the thermal element is fedduring the rest phase (duration T_(rest)). P_(rest)<P_(measurement) isalways true and advantageously T_(rest)>0.5*T_(measurement). It is alsopossible to insert markedly longer phases of rest to save energy.

The process may also be carried out with a plurality of thermalmeasuring elements, for example, pellistors, in which case a pulsedtwo-bead operation can be advantageously embodied.

However, an essential advantage of the process according to the presentinvention sufficient to compensate environmental effects and to reliablyuncouple them from the effect of the target gas proper, whoseconcentration is to be measured.

The present invention will be explained in greater detail on the basisof an exemplary embodiment. The various features of novelty whichcharacterize the invention are pointed out with particularity in theclaims annexed to and forming a part of this disclosure. For a betterunderstanding of the invention, its operating advantages and specificobjects attained by its uses, reference is made to the accompanyingdrawings and descriptive matter in which preferred embodiments of theinvention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a pellistor suitable for carrying out the process;

FIG. 2 is a U-I curve of a pellistor operated according to the presentinvention;

FIG. 3 is a U-t curve of a pellistor operated according to the presentinvention;

FIG. 4 is an I-t curve of a pellistor operated according to the presentinvention; and

FIG. 5 is a lock-in amplifier that may be used for a lock-in process asa feature in combination with the other process features according tothe present invention to increase the accuracy of measurement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in particular, FIG. 1 shows a pellistorsuitable for carrying out the process according to the invention. Thispellistor comprises a platinum coil 1, on which a catalytically coatedbead 2 is located. The platinum coil 1 is connected to contact pins 3,which lead through a base 4.

FIG. 2 shows a U-I curve in the family of characteristics of a pellistoroperated according to the present invention, as it is passed throughduring a pulse. The pellistor is supplied with I_(rest) before thepulse, during the rest phase. This current is sometimes sufficient todetermine the resistance of the pellistor and thus its temperature. Theresistance reached at the end of the rest phase at point A is designatedby R_(cold). The vertical section F-A shows that cooling of thepellistor takes place during the rest phase and the voltage drops withthe resistance at a constantly low I_(rest).

A constant heating current I_(max) is fed at the beginning of themeasurement phase, which causes a jump from A to B in the family ofcharacteristics. This current leads to heating of the pellistor andconsequently to an increase in the electric resistance, illustrated bysection B-C.

The voltage is limited by the circuitry to a maximum U_(max). When thismaximum is reached, the heating output that is the maximum during apulse is reached at the same time at point C. However, the temperatureand hence the resistance continue to increase. As a result, the currentdecreases at constant voltage U_(max) until a preset adjustableresistance R_(hot) is reached at point D.

The current and the voltage are adjusted beginning from point D byadjustment of the constant resistance such that the resistance of thepellistor is maintained constantly at R_(hot). The thermal equilibriumof the pellistor with its environment is set during this phase. Thisprocess takes place on the linear section D-F of the curve. This sectionintersects the family of curves K, K′, which describes the U-Icharacteristic of the pellistor in quasi-stationary operation as afunction of the target gas concentration. The setting of the thermalequilibrium of the pellistor will therefore end in the family ofcharacteristics at the intersection of the linear section D-F with thecharacteristic K, K′ belonging to the particular target gasconcentration. The stationary working point of the pellistor, whichbelongs to a certain temperature and target gas concentration, isreached in this case. Point E marks such a working point in the absenceof the target gas, and point E′ marks a stationary working point at acertain target gas concentration. The measuring phase can be concludedat the latest when the stationary state is reached and the rest phasebegins again.

The essence of this exemplary embodiment is that at least one thermalmeasuring element is heated until a preset resistance R_(hot) isreached, the thermal measuring element is subsequently operated at thispreset resistance by means of a constant resistance control, the currentand voltage values necessary for the controlled operation of the thermalmeasuring element are determined, and stationary end values, which themeasured current and voltage values seek to reach, are determined fromthese measured values.

It is possible without problems to preselect the preset resistanceR_(hot), up to which at least one thermal measuring element is heated,in a gas species-specific manner. The species of a gas present can thusbe determined by carrying out the measurement with different preselectedresistances R_(hot) one after another.

The measurement phase can advantageously be kept shorter if thevariables describing the stationary working point are determined byextrapolation. Measured values that can be obtained during thecontrolled phase after reaching the hot resistance R_(hot) are used tosupport the extrapolation.

In a typical embodiment, the duration of the measuring phaseT_(measurement) is between 10 msec and 3,500 msec and the duration ofthe rest phase T_(rest) is between 50 msec and several seconds.Measuring phases between 100 msec and 1,500 msec are especiallyadvantageous.

An exponential extrapolation is advantageously performed to determinethe stationary end values. The difference between the stationary endvalues of the current (ΔI) and the voltage (ΔU), which is obtained whenone measurement is carried out in the absence of the target gas and onemeasurement is performed at a target gas concentration to be determined,is advantageously used as an indicator of the concentration of a targetgas to be determined.

FIG. 3 and FIG. 4 show the voltage and current curves belonging to theabove-described cycle for a pellistor operated according to the presentinvention as a function of time. Sections B-C and C-D are shown aslinear sections for simplicity's sake. However, all curve sectionshaving a finite rise may have curvatures in practice and be included inthe determination of transient states according to the presentinvention. Section D-E or D-E′ is a range that is of particular interestfor the evaluation. The electric variables U and I asymptoticallyapproach an end value each, which describes the stationary working pointof the pellistor. The target gas concentration can be determined fromthe position of this working point. The measurements necessary for thiscan be terminated when sufficient information or a sufficient amount ofmeasured values that permit the working point to be determined byextrapolation are available.

Various special cases are conceivable when the process according to thepresent invention is carried out.

For example, the maximum current I_(max) fed in may be equal to themeasuring current. The operation then takes place in theconstant-current operation during the pulse. The temperature necessaryfor the operation of the thermal measuring element is generated by meansof a constant current. The measured variable is the voltage, its courseduring the measuring phase and the end value, which becomes establishedat the end of the measuring phase. This can in turn be determined byextrapolation.

In another special case, U_(max) corresponds to the measuring voltage.The operation is then carried out in the constant-voltage operationduring the pulse. The temperature necessary for the operation of thethermal measuring element is generated by means of a constant voltage.The measured variable is the current, its course during the measuringphase and the end value that becomes established at the end of themeasuring phase. This can in turn be determined by extrapolation.

The thermal measuring element is switched off completely during the restphase in a third special case. No measurements are performed.

The current and/or voltage drop over gas-sensitive thermal measuringelements, especially over a pellistor, are used as measured variables inthe process being described, and the course of these variables over timeis determined. Derived variables, e.g., the resistance, the temperature,the electric heating output, the rates of heating and cooling, achemical heating output, a thermal resistance and other physicalvariables can be derived from this and included in evaluations.

The time course of the current and voltage curves and the end valuesthat may possibly become established depend on ambient conditions suchas the ambient temperature, air pressure, relative humidity and the gascomposition. At the same time, information can be obtained on the gascomposition and the ambient conditions by measuring these time courses.

The process, which can be called a transient method according to thepresent invention, can be embodied with different evaluation methods. Itis essential that the evaluation of the response of a thermal measuringelement to a current-voltage pulse fed reveals transient states of thethermal measuring element, from which a prevailing target gasconcentration and interference variables related to the environment canbe unambiguously inferred at the same time.

As a result, only one measuring element is necessary, with which themeasurement proper and a necessary compensation of interference effectscan be carried out. Very good results can be obtained with a pellistor,which has a catalytically coated bead with a volume of less than 2 mm³,at a power consumption of less than 80 mW, and even markedly lower powerconsumptions, e.g., below 10 mW, can be achieved by changing thepulse-duty factor. Reaction temperatures that are below 570° C. can bereached on the catalyst surface for the important target gas methane bymeans of corresponding material combinations, for example, Rh, Pt or Pdas the catalysts and aluminum oxide, zirconium oxide or magnesium oxideas the carrier materials, which are used to support the catalysts. Themeasurement takes place so rapidly and converts so little gas that theprocess becomes independent from transport processes, i.e., thediffusion of products and educts.

For example, the derivations of the current and voltage curves overtime, time integral, frequency analyses or determination of thepercentage of harmonics in the spectrum of the curves recorded can beused to evaluate the recorded curves.

Changes in activity in a catalytically active thermal measuring element,as they may occur due to aging phenomena or chemical poisoning, canadvantageously also be determined with the process according to thepresent invention by determining changes in the transient behavior ofthe thermal measuring element.

To increase the accuracy of measurement, the process according to thepresent invention can be combined with a lock-in process. While thesensor is modulated with a fixed frequency the response is a signalcarrying components of the same frequency and harmonic multiples thereofwith fixed phases related to the modulation. The noise, however, isuncorrelated. A lock-in process is a way to filter the response signalto exactly the same frequency as the modulation signal (or a harmonicmultiple) and a fixed phase in relation to it. The bandwidth passing thelock-in process is a very small fraction of the signal bandwidth so thatthe total noise is reduced to the same fraction. A lock-in amplifier isshown in FIG. 5. This amplifier circuit generally designated 10 isconnected to the control current-voltage pulse (V+ctrl, V−ctrl) andpellistor contacts (V+Pelli, V−Pelli) and includes reistors R1-R4,operational amplifier or integrated circuit IC1 and transistor/amplifierQ2. Although such electronic circuits are quite suitable for thispurpose, a lock-in process can also be performed by software thatprocesses the digitized response signal. If the sensor is heatedperiodically with a frequency close to the inverse of its time constantthen the resistance of the wire will be a periodic function with themodulation frequency and a fixed phase shift very similar to a sinusoidwith little contributions from harmonic frequencies. If a catalyticreaction takes place during the time fractions when the bead is hot theresistance will be influenced by the additional heat. This willcontribute considerably to the amplitudes and phases of the harmonicsignals. A combination of these harmonic amplitudes give a valuerepresentative for the concentration of the reactants.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

1. A process for measuring gas concentration, the process comprising: providing a thermal measuring element; operating said thermal measuring element by applying pulses; and obtaining a measured value of the gas concentration in the environment of the measuring element from the evaluation of a response of the measuring element to at least one single pulse by determining transient states of the thermal measuring element during the imposed pulse, from which transient states the measured value of the gas concentration in the environment of the measuring element can be derived from accessible electric measured variables, the thermal measuring element being heated until a preset resistance is reached, the thermal measuring element being subsequently operated at this preset resistance by means of a constant resistance control, the current and voltage values necessary for the controlled operation of the thermal measuring element being determined, and stationary end values, which the determined current and voltage values seek to reach, are determined from these determined values, the determination of the stationary end values being performed by exponential extrapolation, which is based on measured values of the current and/or voltage, which are obtained during the operation of the thermal measuring element by means of a constant resistance control, exponential extrapolation being performed to determine the stationary end values. 